The ever increasing demand for achieving and maintaining minimum exhaust emissions and fuel efficient internal combustion engines has required increasingly higher fuel injection pressures. These higher fuel injection pressures, particularly in unit injectors in diesel engines, increase the hertz stress on the injector-actuating cam shaft and its associated following mechanism. When the unit injectors and the intake and exhaust valves in a diesel engine are actuated by the same cam shaft, space constraints can severely limit the axial placement of the injector lobes since all of the valve-actuating lobes and the injector-actuating lobes must be located on the same cam shaft. When sufficient axial space is not available, the hertz stresses on the cam shaft can become unacceptably high. Unless hertz stresses on the cam shaft are kept to a reasonable level, injector pressures of a desirable magnitude cannot be achieved.
Cam-operated unit fuel injectors are known in the art. U.S. Pat. No. 5,315,974 to Sabelstrom et al, for example, discloses a diesel engine with a fuel injection system which employs a cam shaft positioned in the engine overhead for operating a unit fuel injector.
Dual overhead cam shaft arrangements are also known. Most dual overhead cam shaft arrangements include one cam shaft dedicated to actuating the intake valves and one cam shaft dedicated to actuating the exhaust valves. U.S. Pat. No. 4,836,171 to Melde-Tuczai et al is illustrative of an internal combustion engine which employs two overhead cam shafts. The dual cam shaft arrangement disclosed in this patent includes cams for separately mounted rocker arms actuating pump nozzles which, in turn, are operated by the two cam shafts. The first cam shaft actuates the pump associated with one cylinder, and the second cam shaft actuates the pump associated with the adjacent cylinder. It is not suggested that this arrangement could be used to actuate unit fuel injectors; rather, it is designed to permit the easy disassembly of a selected pump nozzle without disturbing the two cam shafts and all the remaining valve gear.
None of the art of which applicants are aware suggests dedicating one cam shaft in a dual overhead cam shaft or other cam shaft arrangement solely to the actuation of the engine unit fuel injectors.
The prior art, therefore, has failed to provide a drive train actuation assembly for an internal combustion engine with a dual overhead cam shaft arrangement wherein one of the cam shafts is dedicated solely to actuating the engine unit fuel injectors, thereby minimizing hertz stresses on the cam shaft and prolonging cam life.